The present invention relates to a current source inverter circuit and, more particularly, to a current source inverter circuit of the type suitable for driving an A.C. motor.
Inverters basically to convert a D.C. current to an A.C. current, and are advantageously used for driving A.C. motors at variable speeds because of their variable output frequencies. In driving an A.C. motor with an inverter, it is strictly required to maintain the ratio of voltage to frequency almost constant. For this reason, in general, the inverter for driving an A.C. motor is used in combination with a D.C. source, as a current source inverter circuit.
A typical inverter circuit of this kind is shown in U.S. Pat. No. 3,336,520 in which the inverter is comprised of branches each of which is provided with a pair of thyristors adapted to generate an A.C. output at an intermediate point and diodes provided at the intermediate points, as well as a commutating capacitor provided between the connecting points of the thyristors and the diode in each branch. In this type of inverter, however, the commutating capacitor has not been charged at the time of starting the motor. Therefore, the commutation may be disturbed resulting in a commutating failure.
In order to avoid this failure, attempts have been made to charge the capacitor by temporarily increasing the output frequency at the time of the starting operation, or to enable the current to be commutated by restraining the rising of the output current. However, such attempts inconveniently necessitate complicated controls or prolonged starting time of the motor.
Another solution employing an auxiliary charging circuit for charging the commutating capacitor in the prior art device at the time of starting has been found also to be unacceptable due to the greatly increased complexity of the circuitry involved. This solution is based on the provision of a capacitor between respective adjacent branches.
On the other hand, another problem inherent in the current source inverter circuit is that the over-charging of the capacitor must be strictly avoided especially during a quick start, abrupt deceleration or acceleration, or an abrupt reversing of the motor, as well as in a starting of the motor with a heavy load. In general, the commutating capacitors are designed to have a small capacity, in order to avoid unstable operation with a light load. However, this requirement is quite incompatible with the above-mentioned prohibition against over-charging of the capacitor.
Thus, due to the contradictory requirements of protecting the thyristors and stabilizing the running of the motor, it is a critical problem to determine how best to control the over voltage to the capacitor.
Recent demands requiring a higher response for quick start and abrupt acceleration/deceleration for electric motors driven by current source inverter circuits requires an inverter circuit of a simplified structure which is capable of performing the double functions of preserving the starting inverting voltage for the commutating capacitor as well as prevention of over-voltage in cases of overloads.